1. Field of the Invention
This invention relates generally to a steering system for automotive vehicles. It more particularly relates to a rear wheel steering mechanism which can be advantageously incorporated into a four-wheel steering system (4WS) for automobiles.
2. Description of the Prior Art
As is well known, various attempts have long been made to improve controllability of automobiles. The four-wheel steering system is one example which has resulted from such attempts.
In designing the four-wheel steering system, the following points must be taken into consideration.
(1) High speed driving rarely requires large angle rotation of the steering wheel because such driving is used principally for driving on straight roads or in gentle corners.
(2) Low speed driving, on the other hand, often requires large angle rotation of the steering wheel to make steep curves or U-turns with a small turning radius.
(3) When driving in a corner particularly at high speeds, the car is subjected to a larger centrifugal force which may cause lateral slippage.
(4) Lateral slippage of the car can be prevented or reduced by orienting or pivoting the rear wheels in the same direction as the front wheels Such pivotal movement of the rear wheels will be hereafter called "same phase pivotal movement".
(5) The turning radius of the car can be reduced by orienting or pivoting the rear wheels in the opposite direction relative to the front wheels Such pivotal movement of the rear wheels will be hereafter called "opposite phase pivotal movement".
In view of these points, U.S. Pat. No. 4,295,657 to Sano et al discloses a four-wheel steering system for a vehicle which comprises a rear wheel steering mechanism mechanically connected to a front wheel steering mechanism. More specifically, the rear wheel steering mechanism includes a fixed housing in which is slidably guided a cam plate connected through a link rod to a knuckle arm of one front wheel. The link rod converts the pivotal steering movement of that front wheel into the sliding movement of the cam plate lengthwise of the vehicle. The cam plate is formed with a snaky or tortuous cam groove in which is slidably fitted a guide pin connected through a tie rod to the knuckle arms of the rear wheels. Thus, the movement of the cam plate lengthwise of the vehicle forces the pin to move widthwise of the vehicle, thereby causing the rear wheels to pivot for directional change.
The tortuous cam groove is so shaped that the rear wheels make same phase pivotal movement when the steering wheel is rotated through less than a specified angle, whereas the rear wheels make opposite phase pivotal movement when the steering wheel is rotated through more than the specified angle. Thus, with the four wheel steering system of this U.S. patent, it is possible to achieve a smaller turning radius of the vehicle during low speed driving while ensuring a increased safety during high speed driving.
The four-wheel steering system of the above U.S. patent, however, is disadvantageous in that a wasted large force is developed between the guide pin and the cam groove when converting the movement of the cam plate lengthwise of the vehicle into the movement of the pin or the tie rod widthwise of the vehicle. Further, such a manner of movement conversion may result in damage of the cam plate or the guide pin particularly when the rear wheels to be pivotally moved are heavily loaded.
British Patent Application Laid-open No. 2,123,768 discloses another four-wheel steering system for a vehicle which comprises a rotary member adapted to rotate about a rotational axis extending lengthwise of the vehicle in response to the rotation of the steering wheel. The rotary member carries a crank pin eccentric relative to the rotational axis of the rotary member. The crank pin is connected to a link arm extending widthwise of the vehicle and supported to move pivotally as well as widthwise of the vehicle upon cranking or eccentric rotation of the crank pin. The movement of the link arm widthwise of the vehicle is transmitted to the knuckle arms of the rear wheels to cause steering pivotal movement thereof. Basically, a half rotation of the rotary member corresponds to the same phase pivotal movement of the rear wheels, whereas a further half rotation of the rotary member is equivalent to the opposite phase pivotal movement of the rear wheels. Therefore, a turning point from the same phase pivotal movement to the opposite phase pivotal movement is at the point of half rotation of the rotary member.
With the four-wheel steering system of this laid-open British application, it is possible to change the amount of steering pivotal movement of the rear wheels by varying the eccentricity of the crank pin. However, it is virtually impossible to adjust the amount of the same phase pivotal movement independently of the amount of the opposite phase pivotal movement. Similarly, it is also impossible to adjust the timing of the turning point. Further, the link arm connected to the crank pin is pivotally moved, which is a wasted movement not utilized for steering pivotal movement of the rear wheels.
U.S. Pat. No. 4,522,416 to Sano et al also teaches a four-wheel steering system which is similar to the above British patent and therefore has similar disadvantages.